Method of electrolytically coating magnesium and its alloys



depending upon the extent to which metal loss or dimensional changes dueto the pickling can be tolerated.

Solution A To be used when dimensional changes must be a minimum:

Chromic acid (Cros) pounds-.. 1.5 Sodium nitrate (NaNOs) do 0.25Magnesium fluoride (MgFz) ounce 0.3

Dissolve in sufficient water to make 1 gallon of solution The oilandgrease-free articles to be cleaned are immersed in the solution at 75-85C. forV Solution B To be used Where small dimensional changes can betolerated:

Glacial acetic ac'id pint-- 0.9 Sodium nitrate pound 0.25

AWater to -make l gallon of solution The solution is used at roomtemperature, the articles to be cleaned being immersed therein for a fewminutes until cleaned.

Although most articles can be suitably pickled by either of theforegoing pickling solutions with due regard for the amount of metalloss involved, die castings are generally best pickled in solution A fora short time, usually 15 to 30 seconds.

Ordinary fabricated articles of magnesium and alloys which have notreceived any special treatment and which are free from oil and greasemay be satisfactorily prepared for the coating treatment according tothe invention by conventional mechanical methods, such as machining,hurling, and sand blasting.

The bath used for carrying out the coating treatment according to theinvention may be prepared in various Ways depending upon the choice ofingredients, and the pH adjusted to between about 10 and l12 by theaddition, if necessary, oi an alkaligsuch as sodium hydroxide. In thepreferred embodiment of the invention, a borate, silicate, and phenolare used in aqueous solution at a pH between about l and 12. An exampleof a formulation including the aforementioned types of ingredients andwhich is illustrative of a preferred practice of the invention is thefollowing:

In compounding this solution, a portion of the water needed may beplaced in a tank and the requisite amount of phenol and sodium hydroxideadded with vigorous agitation, then the sodium meta borate and sodiummeta silicate are added together with suiiicient additional water tomake up the required proportions. Sodium phenate may be used in place ofphenol and sodium hydroxide. The tank may be of any suitable material,e. g. magnesium.

In operating the bath, its temperature is maintained preferably betweenabout '70 and 85 F., although somewhat higher or lower temperatures maybe used. A water coil may be used in the tank to control thetemperature, if necessary.

The articles to be coated (after suitable preparation as described ifcleaning be required) are suspended in the solution in the tank. Thearticles are connected to one lead of a source of alternating current asthe electrolyzing potential, the other lead being connected to thesolution through a magnesium electrode immersed in the solution orpreferably through additional articles to be coated which are alsosuspended in the solution so as to function as the other electrode. Inusing two lots of articles as the pair of electrodes, it is desirable tomatch their surface areas, at least approximately, so that the currentdensity of the one electrode (or lot of articles) will be equal to thecurrent density of 'the other electrode (or lots of articles) at leastapproximately. The A. C. source should be capable of supplying up toabout 150 volts in gradual incrementsf'iso that both the current passingthrough the Work and the voltage` across the electrodes may be subjectto appropriate regulation. A.

After immersingin the bath the articles to be coated, the electrolyzingpotential is applied beginning with a4 low voltage, i. e. in the range 0to 35 volts, so as to produce a current flow through the work equal tofrom about 10 to l5 amperes per square foot of exposed surface of thearticles connected to one of the two leads of the source ofelectrolyzing current. In the beginning of the electrolysis, theresistance to the passage of current is small so thatl a fairly smallvoltage suiiices to produce the required current density, Once thecurrent begins to ow an electrically resistant coating begins to form onthe articles and becomes increasingly resistant to current ow as `theelectrolysis is continued. Thus, in order to maintain the requiredcurrent density as the coating is forming, the voltage applied totheelectrodes is increased in accordance with the increase in resistanceso that a more or less constant current flow is maintained (preferablyin the range of about 10 to 15 amperes per square foot of exposedelectrode area as aforesaid) until the desired coating is obtained.

This may be ascertained in various ways, as by xing the duration of theelectrolyzing treatment. For example, the treatment may be allowed torun for from about 5 to 20 minutes. In the shorter time ofV theforegoing range, the

Vcoating obtained is relatively thin and is useful where only a lightcoating is required. In the middle of the range, usually the lmostdesirable coatings are obtained. In the longer time of the range, thecoatings tend to deteriorate. Another Way to determine duration of thetreatment is to terminate the electrolysis when the voltage required tomaintain the proper current density reaches the range of to 150 volts.Undue prolongation of the electrolysis results in a deterioration of thecoating, as alreadi7 indicated, and this fact may be used also as aguide in determining when the electrolysis should be discontinued.Periodic examination of the coating as it forms' will reveal when itsmaximum hardness and abrasion resistance has been attained. It isusually preferable to end the electrolysis at a voltage of about to 120.After the electrolysis treatment has run for the desiredlength oftime,"the current is cut o and 5 then the articles are removed from thesolution and rinsed with water, preferably heated to near boiling, so asto facilitate subsequent drying of the rinsed articles.

Although the foregoing example is typical of a preferred bathcomposition in which to perform the electrolysis, other proportions andingredients may be used withinthe purview of the invention. In general,the concentration of the borate may be from about 80 grams per liter upto and including saturating amounts, the concentration of the silicatebetween about 5 and 100 grams per liter, and the concentration of theorganic compound between about 1 and 50 grams per liter. The preferredproportions are respectively 150 to 300 grams per liter, l0 to 50 gramsper liter, and 5 to 20 grams per liter.

While the bath may be operated without either the silicate or theorganic compound, the presence of silicate increases the hardness of thecoating, while the presence of the organic compound permits highervoltages to be attained for the same current density and more uniform,dense, and harder coatings result.

The following table sets forth further examples of the practice of theinvention and the abrasion resistance of the coatings produced, theabrasion resistance being measured by the Haueisen abrasimeter. In usingthis device, the abrasive was 200 mesh aloxite and the air pressure 12ounces per square inch.

diethylene glycol, mannitol, phenol, sodium phenate, the pH of thesolution being between about 10 and l2 and the electrolyzing potentialbeing raised gradually from an initial value of between and about 35volts so as to produce a current density between about and 40 amperesper square foot of surface of the article during the electrolysis anddiscontinuing the electrolysis when the appiied voltage reaches a valuebetween 80 and 150.

2. The method of producing an abrasion resistant coating upon anuncoated article 'of magnesium and its alloys containing at least 80 percent of magnesium which comprises anodically electrolyzng the article inan aqueous solution the dissolved constituents of which consistessentially of an alkali metal borate in amount between 150 and 300grams per liter, an alkali metal silicate in amount between and 50 gramsper liter, and from 5 to 20 grams per liter of a water-soluble organiccompound selected from the group consisting of methyl alcohol, ethylalcohol, ethylene glycol, diethylene glycol, mannitol, phenol, sodiumphenate, the pH of the solution being between about 10 and 12 and theelectrolyzing potential being raised gradually from an initial value ofbetween 0 and about 35 volts so as to produce a current density betweenabout 5 and 40 amperes per square foot of surface of the article duringthe electrolysis and Table Bath Composition: Grams per 100 Grams of Bath(Balance Being Water) Example Voltage Abrasicn N o, Organic Compound *mdI-Ileslcse NazBOzAHzO NanSOaHzO Kind Amount 100 12. 5 24 5 100 19. 4 247 sod1um phenate 0.5 110 30.7 24 5 I 4 110 36. 1 24 5 9. 7 120 40. 5 245 0.5 140 48. 9 24 5 2 130 84. 1 24 5 l 130 104. 5 24 5 2 140 112. 7

Referring to the table, it will be observed that columns 2 to 5,inclusive, give the composition (in weight per cent) of the aqueoussolution or bath in which the magnesium alloy articles were anodicallyelectrolyzed. The duration of the electrolysis in each example was 10minutes and the voltage was applied gradually at a constant rate ofincrease from 0 at the beginning of the electrolysis until theelectrolysis was terminated at the voltage set forth in column 6. Eachof the coatings obtained was hard, light grey in color, uniform inappearance and possessed the abrasion resistance set forth in column 7.

I claim:

1. The method of producing an abrasion resistant coating upon anuncoated article of magnesium and its alloys containing at least 80 percent of magnesium which comprises anodically electrolyzing the articlein an aqueous solution the dissolved constituents of which consistessentially of an alkali metal borate in amount between 80 grams perliter and that producing saturation, an alkali metal silicate in amountbetween 5 and 100 grams per liter, and from 1 to grams per liter of awater-soluble organic compound selected from the group consisting ofmethyl alcohol, ethyl alcohol, ethylene glycol,

discontinuing the electrolysis when the applied voltage reaches valuebetwPfn and 150.

HERBERT K. DE LONG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Hackhs Chemical Dictionary, 2dedition, 1937, page 737.

Ser. No. 268,694, Berthier (A. P. C.), published June 8, 1943.

1. THE METHOD OF PRODUCING AN ABRASION RESISTANT COAING UPON AN UNCOATEDARTICLE OF MAGNESIUM AND ITS ALLOYS CONTAINING AT LEAST 80 PER CENT OFMAGNESIUM WHICH COMPRISES ANODICALLY ELECTROLYZING THE ARTICLE IN ANAQUEOUS SOLUTION THE DISSOLVED CONSTITUENTS OF WHICH CONSIST ESSENTIALLYOF AN ALKALI METAL BORATE IN AMOUNT BETWEEN 80 GRAMS PER LITER AND THATPRODUCING SATURATION, AN ALKALI METAL SILICATE IN AMOUNT BETWEEN 5 AND100 GRAMS PER LITER, AND FROM 1 TO 50 GRAMS PER LITER OF A WATER-SOLUBLEORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF METHYL ALCOHOL,ETHYL ALCOHOL, ETHYLENE GLYCOL, DIETHYLENE GLYCOL, MANNITOL, PHENOL,SODIUM PHENATE, THE PH OF THE SOLUTION BEING BETWEEN ABOUT 10 AND 12 ANDTHE ELECTROLYZING POTENTIAL BEING RAISED GRADUALLY FROM AN INITIAL VALUEOF BETWEEN 0 AND ABOUT 35 VOLTS SO AS TO PRODUCE A CURRENT DENSITYBETWEEN ABOUT 5 AND 40 AMPERES PER SQUARE FOOT OF SURFACE OF THE ARTICLEDURING THE ELECTROLYSIS AND DISCONTINUING THE ELECTROLYSIS WHEN THEAPPLIED VOLTAGE REACHES A VALUE BETWEEN 80 AND 150.